And say, “The truth is from your Lord, so whoever wills – let him believe; and whoever wills – let him disbelieve.” Indeed, We have prepared for the wrongdoers a fire whose walls will surround them. And if they call for relief, they will be relieved with water like murky oil, which scalds [their] faces. Wretched is the drink, and evil is the resting place. [18:29]

It means: Say to the people, O Muhammad: this is the truth from your Lord. Meaning: Guidance has been clearly distinguished from misguidance, as well as direction from misdirection, as well as the characteristics of the people of ultimate joy and the characteristics of the people of ultimate sorrow. That is what has been clarified by Allah on the tongue of His messenger. So when something has been clarified and has become clear, then no doubt remains concerning it.

فَمَنْ شَاءَ فَلْيُؤْمِنْ وَمَنْ شَاءَ فَلْيَكْفُرْ

meaning: so nothing remains except to traverse one of these two paths, according to the success of the slave or the lack thereof. And Allah has given free-will concerning this for one to decide between al-eemaan and al-kufr, good and evil. So however believes, then he has successfully coincided with what is correct. And whoever disbelieves, then he has established a proof against himself. And there is no forcing or compulsion of al-eemaan, just as Allah said,

لَا إِكْرَاهَ فِي الدِّينِ ۖ قَد تَّبَيَّنَ الرُّشْدُ مِنَ الْغَيِّ

There shall be no compulsion in [acceptance of] the religion. The right course has become clear from the wrong. [2:256]

And His statement,

فَمَن شَاءَ فَلْيُؤْمِن وَمَن شَاءَ فَلْيَكْفُرْ‌

does not entail permissibility of either of the two choices; it is only an intimidation and threat to whoever would choose al-kufr after a complete elucidation, just as there is not an abandonment of fighting the disbelievers in this ayah. Then Allah mentioned the outcome of the two groups…

[Taysir al-Kareem al-Rahman pg. 552-553]

And in his book of tafsir, Sheikh Muhammad al-Ameen al-Shinqitee also wrote:

فَمَنْ شَاءَ فَلْيُؤْمِنْ وَمَنْ شَاءَ فَلْيَكْفُرْ

what is apparent is to take this noble ayah according to the linguistic context as offering a free choice (of preference) between al-kufr and al-eemaan, however the intended meaning of this noble ayah is not the offering of a free choice; the intended meaning of it is only that of impending threat and intimidation. And impending threat with this type of wording which outwardly appears as an offering of a free choice is one of the stylistic forms of the Arabic language.

Indeed, We have prepared for the wrongdoers a fire whose walls will surround them. And if they call for relief, they will be relieved with water like murky oil, which scalds [their] faces. Wretched is the drink, and evil is the resting place.

And this is the most clear proof that the intended meaning is one of threat and intimidation, for if it were the offering of a free choice (of preference), then He would not have promised the chooser of one of these two optional paths with this painful punishment. And this is clear, as you can see.